This invention pertains to laser devices for using laser beams to output prescribed light to expose semiconductors, polymer materials, and inorganic materials and for outputting laser beams for machining apparatuses that perform machining processes, and more particularly to improvements therein in order to obtain constantly uniform pulse energy values when implementing burst mode operations wherein a continuous oscillation action for generating pulses of laser light a prescribed number of times continuously is alternated over and over with a stopping action for stopping the pulse generation for a prescribed time.
In the field of semiconductor exposure devices which employ UV light beams, it is found necessary to implement fine precision exposure light quantity control in order to maintain circuit pattern resolution above a certain level. However, with the excimer lasers that are used as the light sources in these semiconductor exposure devices, there is variation in the pulse energy from one pulse to the next because they are so-called discharge excitation gas lasers. There is a need to reduce this variation in order to improve the precision of exposure light quantity control.
Semiconductor exposure devices, on the other hand, alternately repeat exposures with stage movements. More specifically, in FIG. 9 is diagrammed a semiconductor wafer W whereon are arranged a plurality of IC chips 90, and, when performing exposures with a stepper, upon the completion of exposure processing wherein one IC chip 90 on the semiconductor wafer W has been irradiated with a plurality of continuous light pulses, either the wafer W or the optical system is shifted so that the next unirradiated IC chip 90 can be irradiated with continuous light pulses, whereupon, after this stage shift, light irradiation is performed as before. Upon the completion of this exposure operation on all of the IC chips 90 on the semiconductor wafer W, alternating the exposures and the stage shifts in this manner, the completely exposed wafer W is carried away and the next wafer W is placed in the irradiation position so that the same light irradiation process can be repeated again.
Thus a semiconductor exposure device is fashioned so that exposures and stage shifts are alternately repeated. The operation of an excimer laser constituting the light source in such an exposure device, therefore, as diagrammed in FIG. 10, involves a burst mode operation wherein a continuous pulse generation operation for pulse-generating laser beams continuously a prescribed number of times is repeated together with an oscillation stop interval t during which pulse generation is suspended for a prescribed time interval.
More specifically, in the burst mode operation diagrammed in FIG. 10, the oscillation stop interval t corresponds to the time required to move the stage in the semiconductor exposure device. This oscillation stop interval t, however, for various reasons, is not necessarily constant. When one wafer is being exchanged with another, for example, the oscillation stop interval will be much longer than when moving the stage between IC chips. Also, the oscillation stop interval needed when shifting between IC chips in the same row will be very different from the oscillation stop interval needed when shifting from one IC chip to another IC chip in a different row. When the number or arrangement of the IC chips on the wafer changes, moreover, that will also cause the oscillation stop intervals to change. There are various other factors that cause changes in the oscillation stop interval. It should be noted also that, in FIG. 10, the energy intensity of each pulse is represented for a case where the excitation intensity (discharge voltage) is fixed at a constant value.
In such burst operations as this, when the length of the oscillation stop interval t varies, these variations cause large changes in the output of individual laser pulses, as diagrammed in FIG. 11. More specifically, when the oscillation stop interval t is short, the effects of past laser generations remain in the form of rises in gas temperature, disruption of the gas or gases inside the laser chamber, and localized rises in electrode temperature, etc. When the oscillation stop interval t is long, on the other hand, the effects of past laser generations on the laser disappear. For this reason, even if the laser discharge voltage is held constant, as is diagrammed in FIG. 11, when the stop time is short the output energy will be smaller, and when the stop time is long the output energy will be larger. Thus the laser output will change greatly in response to the oscillation stop interval.
In the meantime, as noted above, an excimer laser is a pulse discharge excitation gas laser, for which reason it is very difficult to continue oscillations so as to produce a pulse energy that is always at a constant level. There are at least two reasons for this, namely (1) density commotion in the laser gas inside the discharge space develops due to the discharges, making the next discharge uneven and unstable, and (2) localized temperature rises occur in the surface of the discharge electrodes due to these uneven discharges, etc., which result in deterioration in the next discharge and cause discharges to be uneven and unstable.
This tendency is particularly pronounced during the initial stage of the continuous pulse generation interval described above. As diagrammed in FIG. 12, in the spike region that contains the first several pulses after the completion of the oscillation stop interval t, at first comparatively high pulse energy is obtained, but thereafter the pulse energy gradually falls. This is the so-called spiking phenomenon. When this spike region is finished, the pulse energy passes through a plateau region wherein a stable value continues at a comparatively high level, and then enters a steady region.
Thus, with an excimer laser device operated in burst mode, the energy variation between pulses described above causes the precision of quantitative exposure control to decline, and the spiking phenomenon makes this variation even more pronounced, resulting in a large decline in quantitative exposure control, which is a problem.
In the face of this problem, the applicant has filed for patents on various inventions pertaining to so-called spike prevention control wherein, using the property whereby the energy of pulses generated increases as the excitation intensity (charging voltage, discharge voltage) increases, the discharge voltage (charging voltage) for the first pulse in continuous pulse generation in the burst mode is made smaller, and the discharge voltage for the following pulses is made gradually larger, thereby changing the discharge voltage for each pulse and preventing the initial energy rise due to the spiking phenomenon (Japanese Patent Application No. 4-191056, Japanese Patent Application Laid-open No. 7-106678 (Japanese Patent Application No. 5-49483), etc.).
More specifically, as based on the prior art cited above, discharge voltage data for causing the energy of each pulse in continuous pulse generation to be at a desired target value Pr, taking various parameters such as oscillation stop interval t and power lock voltage (the power supply voltage determined in response to the deterioration of the laser gas) into consideration, are stored beforehand in a table for each pulse in the continuous pulse generation, the pulse energy Pi (where i=1, 2, . . . ) for the current continuous pulse generation is detected, this detected value Pi is compared against the pulse energy target value Pd, and, based on the results of this comparison, the discharge voltage data for each pulse previously stored, as noted above, are corrected and updated. These corrected voltage data are used as the discharge voltage data during the next burst cycle.
In the discharge voltage correction control described above, the pulse energy Pi resulting from laser oscillation is detected using the discharge voltage datum Vi stored in the table noted above, the difference xcex94P (=Pixe2x88x92Pr) with the target energy Pr is computed, a discharge voltage correction value xcex94V (=Gxc2x7xcex94P where G is the gain constant) is computed according to the difference xcex94P, the discharge voltage datum Vi recorded in the aforesaid table is corrected using this discharge voltage correction value xcex94V, and thus a post-correction discharge voltage datum Vixe2x80x2 (=Vi+xcex94V) is obtained.
As based on the conventional correction control described in the foregoing, however, the gain constant G is fixed at the same value in all regions (i.e. the spike region, plateau region, and steady region) in the burst cycle diagrammed in FIG. 12, wherefore the effectiveness in suppressing variation in pulse energy is not adequate.
More specifically, looking at FIG. 13, which diagrams the relationship between excimer laser discharge voltage and pulse light power, based on these characteristics, a discharge is generated at a voltage Vc or higher and laser oscillation occurs. While the voltage is low, the pulse light power and the voltage are roughly proportional. When the voltage rises, however, saturation occurs, and the rise in pulse light power associated with a rise in voltage decreases. In FIG. 13, the change in voltage required to effect the same rise in power xcex94P is represented in two types according to the size of the discharge voltage. Between the change xcex94VL when the voltage is low and the change xcex94VH when the voltage is high, the relationship xcex94VL less than xcex94VH clearly holds.
As based on the spiking prevention control described in the foregoing, on the other hand, the discharge voltage for the first several pulses is made small, and thereafter the discharge voltage is gradually made larger, so that, as a result, the discharge voltage fluctuates largely in units corresponding to the region (i.e. the spike region, plateau region, and steady region).
Thus, with the conventional spiking prevention control, despite the large fluctuations in units corresponding to the discharge voltage units of spike region, plateau region, and steady region, the gain constant G is fixed at the same value, as noted, wherefore the variation in pulse energy cannot be adequately suppressed to the level required in a semiconductor exposure device.
As based on the prior art described in the foregoing, moreover, spike killer control is implemented in the plateau region and steady region in addition to the spike region diagrammed in FIG. 12, wherefore the effectiveness in suppressing pulse energy variation in regions other than the spike region is inadequate.
The explanation for this is thought to be that, with the initial pulses of the continuous pulses, the effects of stopping laser oscillation (i.e. causing the laser to stabilize) remain strongly, so that, even when the same discharge voltage is applied, the output power therefrom is large compared to the other regions, whereas in the following plateau region and stable region, the effects of stopping laser oscillation become smaller, and the pulse generation effects up until immediately prior thereto (i.e. rises in electrode temperature, laser gas disturbance, etc.) are more strongly sustained.
With the prior art described in the foregoing, furthermore, spike killer control is implemented for all pulses produced by the continuous pulse generation, wherefore the volume of data in memory becomes large, so that not only is enormous memory capacity required, but considerable time is needed to read the data out of memory. These are problems.
The present invention has been devised in view of the situation described above. An object of the present invention, therefore, is to provide a laser device wherewith the pulse energy of all pulses produced by continuous pulse generation is made continually, uniform, and both laser light and laser machining precision are further improved.
The present invention provides a laser device which repeatedly performs a burst mode operation, wherein an operation of alternately implementing continuous oscillation actions to pulse-generate laser light continuously a prescribed number of times and stopping actions for stopping the pulse generations only during prescribed oscillation stop intervals constitutes one burst cycle, and which controls a laser power supply voltage so that energy of each output of the pulse generation falls within a prescribed target value range, characterized in that the laser device comprises:
voltage data table means for, taking pulse numbers indicating order of pulses in one burst cycle and a plurality of different oscillation stop intervals as parameters, recording beforehand initial values for the power supply voltage, making output of each of the pulse generations a value within a prescribed allowable value range near the target value, respectively; control gain setting means for dividing each burst cycle containing a plurality of pulses into a plurality of blocks, setting control gains used when correcting the power supply voltage values stored in the power supply voltage data table means to values that differ by block units so that those values are smaller the smaller are the pulse numbers of the pulses contained in the block, dividing the oscillation stop intervals into pluralities of blocks according to size of the intervals, and setting control gains to values that differ by block units so that those values are smaller the larger are the oscillation stop intervals contained in the blocks; oscillation stop interval measuring means for measuring the oscillation stop intervals in each burst cycle;
oscillation control means for reading out power supply voltage values from the power supply voltage data table means corresponding to measured oscillation stop intervals and corresponding to the pulse numbers, each burst cycle, and performing pulse generation in accordance with the power supply voltage values read out; monitor means for associating outputs of the pulses continuously generated with the pulse numbers and monitoring order thereof; and table correction means for finding, for each pulse, differences between output values of the pulses monitored by the monitor means and the target values, and, for pulses for which this difference exceeds an allowable limit, correcting and updating the power supply voltage values stored in the voltage data table means corresponding to pulse numbers of those pulses and the oscillation stop intervals measured, using the differences and the control gains of the control gain setting means set in the blocks corresponding to those pulse numbers and to the oscillation stop intervals measured.
As based on the invention so described, when correcting the power supply voltage data stored in the power supply voltage data table, the control gains used in making these corrections are divided into groups corresponding to pulse numbers and oscillation stop intervals, and different values are set in units of these groups. Furthermore, these grouped control gains are set with different values in block units so that, for blocks divided according to pulse number, the values become smaller as the pulse numbers contained in the blocks become smaller, while, for blocks divided according to oscillation stop interval, the values become smaller as the intervals become larger.
That being so, as based on the present invention, power supply voltage control is implemented such that the relationship between the power supply voltage and the pulse light power which was not linear becomes a pseudo-linear relationship, and it becomes possible to make the output of all pulses uniform with high precision even when the oscillation stop interval changes in various ways, so that the exposure light and optical machining precision can be improved even further.
As based on the present invention, moreover, a laser device which repeatedly performs a burst mode operation, wherein an operation of alternately implementing continuous oscillation actions to pulse-generate laser light continuously a prescribed number of times and stopping actions for stopping the pulse generations only during prescribed oscillation stop intervals constitutes one burst cycle, and which controls a laser power supply voltage so that energy of each output of the pulse generation falls within a prescribed target value range, characterized in that the laser device comprises:
voltage data table means for, taking pulse numbers indicating order of pulses in one burst cycle and a plurality of different oscillation stop intervals as parameters, recording beforehand initial values for the power supply voltage, and making output of each of the pulse generations a value within a prescribed allowable value range near the target value; control gain setting means for dividing each burst cycle containing a plurality of pulses into a plurality of blocks, and setting control gains used when correcting the power supply voltage values stored in the power supply voltage data table means to values that differ by block units so that those values are smaller the smaller are the pulse numbers of the pulses contained in the blocks; oscillation stop interval measuring means for measuring the oscillation stop intervals in each burst cycle; oscillation control means for reading out power supply voltage values from the power supply voltage data table means corresponding to measured oscillation stop intervals and corresponding to the pulse numbers, each burst cycle, and performing laser oscillation in accordance with the power supply voltage values read out; monitor means for associating outputs of the pulses continuously generated with the pulse numbers and monitoring order thereof; and table correction means for finding, for each pulse, differences between output values of the pulses monitored by the monitor means and the target values, and, for pulses for which this difference exceeds an allowable limit, correcting and updating the power supply voltage values stored in the voltage data table means corresponding to pulse numbers of those pulses and to the oscillation stop intervals measured, using the differences and the control gains of the control gain setting means set in the blocks corresponding to those pulse numbers.
As based on the invention so described, when correcting the power supply voltage data stored in the power supply voltage data table, the control gains used in making these corrections are divided into groups corresponding to pulse numbers, and different values are set in units of these groups. Furthermore, these grouped control gains are set with different values in block units so that the values become smaller as the pulse numbers contained in the blocks become smaller.
That being so, as based on this invention, power supply voltage control is implemented such that the relationship between the power supply voltage and the pulse light power which was not linear becomes a nearly pseudo-linear relationship, and it becomes possible to make the output of all pulses uniform with high precision, so that the exposure light and optical machining precision can be improved even further.
The present invention further provides a laser device which repeatedly performs a burst mode operation, wherein an operation of alternately implementing continuous oscillation actions to pulse-generate laser light continuously a prescribed number of times and stopping actions for stopping the pulse generations only during prescribed oscillation stop intervals constitutes one burst cycle, and which controls a laser power supply voltage so that energy of each output of the pulse generation falls within a prescribed target value range, characterized in that the laser device comprises:
voltage data table means for, taking pulse numbers indicating order of pulses in one burst cycle and a plurality of different oscillation stop intervals as parameters, recording beforehand initial values for the power supply voltage, and making output of each of the pulse generations a value within a prescribed allowable value range near the target value, respectively; control gain setting means for dividing the oscillation stop intervals into a plurality of blocks according to size of those intervals, and setting control gains used when correcting the power supply voltage values stored in the power supply voltage data table means to values that differ by block units so that those values are smaller the larger are the oscillation stop interval contained in the blocks; oscillation stop interval measuring means for measuring the oscillation stop intervals in each burst cycle; oscillation control means for reading out power supply voltage values from the power supply voltage data table means corresponding to measured oscillation stop intervals and corresponding to the pulse numbers, each burst cycle, and performing pulse oscillation in accordance with the power supply voltage values read out; monitor means for associating outputs of the pulses continuously generated with the pulse numbers and monitoring order thereof; and table correction means for finding, for each pulse, differences between output values of the pulses monitored by the monitor means and the target values, and, for pulses for which this difference exceeds an allowable limit, correcting and updating the power supply voltage values stored in the voltage data table means corresponding to pulse numbers of those pulses and to the measured oscillation stop intervals, using the differences and the control gains of the control gain setting means set in the blocks corresponding to the oscillation stop intervals measured.
As based on the invention so described, when correcting the power supply voltage data stored in the power supply voltage data table, the control gains used in making these corrections are divided into groups corresponding to oscillation stop intervals, and different values are set in units of these groups. Furthermore, these grouped control gains are set with different values in block units so that the values become smaller as the oscillation stop intervals contained in the blocks become larger.
That being so, as based on this invention, power supply voltage control is implemented such that the relationship between the power supply voltage and the pulse light power which was not linear becomes a nearly pseudo-linear relationship, and it becomes possible to make the output of the pulses uniform with high precision, even when the oscillation stop interval has changed in various ways, so that the exposure light and optical machining precision can be improved even further.